Wire up PM.getInstalledApplicationsAsUser(flags, userId) as hidden API

Settings needs to access a variant of getInstalledApplications() which
takes a |userId| argument. Since this is not currently exposed by
PackageManager, Settings calls into PackageManagerService directly. This
is ugly and breaks the regular abstraction layer hierarchy.

The CL fixes the problem by exposing the required variant of
getInstalledApplications() as a hidden API, analogously to what was done
before with getInstalledPackages().

Bug: 32692748
Test: Will be CTS-verifier-tested together with Settings
Change-Id: Id9c4e8e18524d312159821f1a4d5527263c7e950